Download Polarity

Polarity
Polarity
• A polar molecule has charged ends, + and –
(just like poles on a magnet)
• The polarity of a bond is the function of the
difference in electronegativity between two
bonded atoms
• Not every polar bond results in a polar molecule
• Polarity is largely determined by molecular
geometry
Electronegativity
• The measure of the attraction an atom has for
the electrons in a bond
• Pauling scale:
▫
▫
▫
▫
Fluorine assigned a value of 4.0 (highest EN)
All other elements are listed relative to this value
Nonmetals greater than 2
Metals less than 2
Electronegativity Cont.
• Atoms with different electronegativities pull on
the bonding electrons differently
• This results in an uneven distribution of
electrons and therefore, a polar bond.
• These polar bonds could result in a polar
molecule
Nonpolar bonds
• If both bonding atoms have identical EN’s, the
bond is NONPOLAR
• All diatomic elements make nonpolar bonds
and since there are only two atoms in the
molecule the molecule is also nonpolar
Polar bonds
• If the bonding atoms have different EN’s, the
resulting covalent bond will be polar
• The bond is called a polar covalent bond and is
referred to as a dipole
• Ex: H-F
EN of F: 4.0
EN of H: 2.1
Difference: 1.9
Polar molecules
• If there are only two atoms in the molecule and
they are different elements, the molecule is
polar
• If there are only two atoms in the molecule and
they are the same, the molecule is nonpolar
Polar molecules
• If there are more than two atoms in the
molecule, a bond being polar may or may not
result in the entire molecule being polar
• The geometry of the molecule must be
considered
Polarity
• If the same atoms are covalently bonded to the
central atom and there are no lone pairs on the
central atom, the molecule will be nonpolar
• The dipoles on the polar bond are pulled in
equal but opposite directions and therefore
cancel each other out.
Nonpolar molecule
Polarity
• If all the atoms bonded to the central atom are
not the same, or do not have the same EN, then
the molecule will be polar.
• The individual dipoles will not cancel each other
out
Polar molecule
• Ex:
CH3F
Polarity
• Lone pairs on the central atom tend to result in a
polar molecule.
• The lone pair distorts the symmetry of the
molecule
• The individual dipoles will not cancel each other
out
• Exceptions: linear (XeF2) and square planar
geometries (XeF4)
Polar molecule
• Ex: H2O
Polarity
• Hydrocarbons are always nonpolar
• Short chain (1-3 carbons) alcohols (-OH group)
are usually polar
• The carbon chain portion of the alcohol is
nonpolar, so as the chain grows longer, the
polarity diminishes.
The polarity truth!
• The polarity of a bond is a continuum rather
than an either/or situtation
• Bonds are classified as either covalent
(nonpolar), polar covalent, or ionic based on the
difference in electronegaitvity.
Polarity continuum
• An EN difference of 0.0 is considered nonpolar
covalent
• Both of the atoms share the electrons equally
Polarity continuum
• An EN difference less than 1.7 is considered a
polar covalent bond.
• Ex: H-Cl
• The Cl has the electrons the majority of the time,
but the H is not considered to have “lost” its
valence electrons
Polarity continuum
• An EN difference greater than 1.7 is considered
and ionic bond.
• Ex: Na-Cl
▫ The Cl has the electrons the huge majority of the
time and Na is considered to have “lost” its
valence electrons
▫ Only in an aqueous solution the sodium is Na+ and
chlorine is Cl-